In a reception device which receives differential voltage signals, a common-mode voltage offset which depends on the transmission device and on the state of the transmission line, and a broad input common-mode voltage to accommodate a low-voltage interface, are sought. Such a reception device is provided with a signal conversion circuit to convert the common-mode voltage of input differential signals into a prescribed voltage level. For example, in an LVDS (Low-Voltage Differential Signaling) reception device which transmits and receives digital signals by changing the current direction in a pair of resistance-terminated differential transmission lines, a signal conversion circuit converts the input common-mode voltage into a voltage which is higher than the sum of a prescribed offset voltage plus the threshold voltage (Vthn) of an NMOS transistor, in order to cause a later-stage high-speed NMOS differential amplifier to operate. Further, in electronic equipment in which lower power supply voltages are being adopted, so-called rail-to-rail operation of circuits is required in order to secure a dynamic range.
A rail-to-rail input signal conversion circuit described in Patent Reference 1 comprises a first differential amplifier circuit, in which an n-type transistor receives input signals, and a second differential amplifier circuit, in which a p-type transistor receives input signals. In this signal conversion circuit, by operating the first amplifier circuit with the input voltage level in a region higher than a prescribed threshold voltage, and by operating the second amplifier circuit with the input voltage level in a region lower than a prescribed threshold voltage, there is mutual compensation, and rail-to-rail input operation is realized.    Patent Reference 1: Japanese Patent Laid-open No. 2000-114892
In the amplifier circuit described in Patent Reference 1, because the operating speed is constrained by the transistor load capacitance, there is the problem that high-speed circuit operation is difficult to achieve. Further, two differential amplifier circuits must be prepared in order to convert the input common-mode voltage, and so there are the problems of an increase in circuit area and greater current consumption.
Hence this invention provides a signal conversion circuit which can decrease the circuit area and reduce current consumption, and which enables faster operation.